1. Field of the Invention
This invention relates generally to the field of semiconductor devices, and more particularly, to a multi-layer dielectric structure and semiconductor devices employing the multi-layer dielectric structure and a method of manufacturing the same.
2. Description of the Related Art
With each generation of metal oxide semiconductor (MOS) integrated circuit (IC), the device dimensions have been continuously scaled down to provide for high-density and high-performance devices. Particularly, the thickness of gate dielectrics is made as small as possible because the drive current in a MOS field effect transistor (FET) increases with decreasing gate dielectric thickness. Thus, it has become increasingly important to provide extremely thin, reliable, and low-defect gate dielectrics for improving device performance.
For decades, a thermal oxide layer, e.g. silicon dioxide (SiO2), has been used as the gate dielectrics because the silicon dioxide thermal oxide layer is stable with the underlying silicon substrate and the fabrication process is relatively simple.
However, because the silicon dioxide has a low dielectric constant (k), e.g., 3.9, further scaling down of silicon dioxide gate dielectric has become more and more difficult. For example, if the thickness of the silicon dioxide gate dielectric is less than 40 angstroms, direct tunneling may occur. As a result, a gate-to-channel leakage current through thin silicon dioxide gate dielectrics increases, leading to an undesirable power consumption problem.
These problems lead to consideration of alternative dielectric materials that can be formed in a thicker layer than silicon dioxide but still produce the same or better device performance. The performance can be expressed as “equivalent oxide thickness (EOT).”
Various attempts have been made to improve the device characteristics of the dielectric materials. For example, U.S. Pat. No. 6,020,024 discloses an oxynitride layer interposed between a silicon substrate and a high-k dielectric layer. U.S. Pat. No. 6,013,553 discloses a zirconium oxynitride layer or a hafnium oxynitride layer as the gate dielectrics. Further, PCT International Patent Application Publication No. WO 00/01008 discloses SiO2, silicon nitride and oxynitride interface layers. Also, U.S. Pat. No. 6,020,243 discloses a high permittivity zirconium (or hafnium) silicon-oxynitride gate dielectrics.
However, such attempts have not succeeded in solving the problems associated with the conventional dielectric materials. For example, the silicon nitride layer or oxynitride layer between the high-k dielectric layer and the silicon substrate or the polysilicon gate electrode causes charge trapping with high interface state densities, thereby reducing channel mobility and also degrading device performance. Further, the formation of the silicon nitride layer or the oxynitride layer requires a relatively large thermal budget.
Accordingly, a need still remains for an improved dielectric layer structure and the manufacturing method to improve the device performance by, for example, reducing the equivalent oxide thickness of the dielectric layer as well as improvement of the interface characteristics.